Conventionally, a ferrite magnet, which is a permanent magnet, has been mainly used for a magnet molded material used in a motor and the like. In recent years, however, an amount of used rare earth magnet with better magnet characteristics has increased in response to high performance and reduced size of a motor.
A rare earth magnet, particularly a rare earth element-iron-boron-base magnet, has been widely used for voice coil motors (VCM) of hard disk drives, magnetic circuits of magnetic resonance imaging (MRI), and the like. In recent years, the applicability has been expanded to driving motors of electric cars. In particular, heat resistance is required for automotive applications, and a magnet having high magnetic characteristics (coercive force (Hcj)) is required to avoid high-temperature demagnetization at an environmental temperature of 150 to 200° C.
An Nd—Fe—B base sintered magnet has a microstructure in which a principal phase of an Nd2Fe14B compound and the like is surrounded by an Nd-rich crystal grain boundary phase (grain boundary phase), and the component composition, size and the like of the principal phase and grain boundary phase play important roles in exerting a coercive force of a magnet. In general sintered magnets, high coercive forces are exerted by containing about a few percent by weight to ten percent by weight of Dy or Tb in a magnet alloy and taking advantages of magnetic properties of a Dy2Fe14B or Tb2Fe14B compound having an anisotropic magnetic field larger than that of the Nd2Fe14B compound. However, there has been a problem in that saturation magnetization is decreased sharply, thereby reducing remanent magnetic flux density (Br), as the content of Dy or Tb is increased. Furthermore, since Dy and Tb are rare resources and are expensive metals costing a few times as much as Nd does, the usage thereof must be reduced.
In order to improve coercive force of Nd—Fe—B base sintered magnet while a decrease in remanent magnetic flux density is suppressed, grain boundary modification technique has been studied such as a grain boundary diffusion method in which rare earth elements such as Dy and Tb are unevenly distributed in a crystal grain boundary phase surrounding the principal phase of Nd2Fe14B compound and the like. The grain boundary diffusion method is a technique which increases coercive force with a small amount of Dy by diffusing dysprosium fluoride and the like from the surface of a sintered magnet along the crystalline grain boundary and increasing crystal magnetic anisotropy of a thin layer in a crystalline grain boundary portion.
WO 2006/043348 A (corresponding to US 2011/0150691 A) discloses a grain boundary diffusion method which uses an oxide and fluoride, which are relatively cheap, among rare earth elements as a diffusing agent. Specifically, the method is a method for producing a rare earth permanent magnet material which comprises disposing a powder containing an oxide or fluoride of Dy or Tb on the surface of a magnet material, and heat-treating the magnet material and the powder at a temperature equal to or below the sintering temperature of the magnet in vacuum or in an inert gas.